mirror of
https://github.com/autc04/Retro68.git
synced 2024-12-11 19:49:32 +00:00
134 lines
3.7 KiB
Go
134 lines
3.7 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
package binary
|
|
|
|
// This file implements "varint" encoding of 64-bit integers.
|
|
// The encoding is:
|
|
// - unsigned integers are serialized 7 bits at a time, starting with the
|
|
// least significant bits
|
|
// - the most significant bit (msb) in each output byte indicates if there
|
|
// is a continuation byte (msb = 1)
|
|
// - signed integers are mapped to unsigned integers using "zig-zag"
|
|
// encoding: Positive values x are written as 2*x + 0, negative values
|
|
// are written as 2*(^x) + 1; that is, negative numbers are complemented
|
|
// and whether to complement is encoded in bit 0.
|
|
//
|
|
// Design note:
|
|
// At most 10 bytes are needed for 64-bit values. The encoding could
|
|
// be more dense: a full 64-bit value needs an extra byte just to hold bit 63.
|
|
// Instead, the msb of the previous byte could be used to hold bit 63 since we
|
|
// know there can't be more than 64 bits. This is a trivial improvement and
|
|
// would reduce the maximum encoding length to 9 bytes. However, it breaks the
|
|
// invariant that the msb is always the "continuation bit" and thus makes the
|
|
// format incompatible with a varint encoding for larger numbers (say 128-bit).
|
|
|
|
import (
|
|
"errors"
|
|
"io"
|
|
)
|
|
|
|
// MaxVarintLenN is the maximum length of a varint-encoded N-bit integer.
|
|
const (
|
|
MaxVarintLen16 = 3
|
|
MaxVarintLen32 = 5
|
|
MaxVarintLen64 = 10
|
|
)
|
|
|
|
// PutUvarint encodes a uint64 into buf and returns the number of bytes written.
|
|
// If the buffer is too small, PutUvarint will panic.
|
|
func PutUvarint(buf []byte, x uint64) int {
|
|
i := 0
|
|
for x >= 0x80 {
|
|
buf[i] = byte(x) | 0x80
|
|
x >>= 7
|
|
i++
|
|
}
|
|
buf[i] = byte(x)
|
|
return i + 1
|
|
}
|
|
|
|
// Uvarint decodes a uint64 from buf and returns that value and the
|
|
// number of bytes read (> 0). If an error occurred, the value is 0
|
|
// and the number of bytes n is <= 0 meaning:
|
|
//
|
|
// n == 0: buf too small
|
|
// n < 0: value larger than 64 bits (overflow)
|
|
// and -n is the number of bytes read
|
|
//
|
|
func Uvarint(buf []byte) (uint64, int) {
|
|
var x uint64
|
|
var s uint
|
|
for i, b := range buf {
|
|
if b < 0x80 {
|
|
if i > 9 || i == 9 && b > 1 {
|
|
return 0, -(i + 1) // overflow
|
|
}
|
|
return x | uint64(b)<<s, i + 1
|
|
}
|
|
x |= uint64(b&0x7f) << s
|
|
s += 7
|
|
}
|
|
return 0, 0
|
|
}
|
|
|
|
// PutVarint encodes an int64 into buf and returns the number of bytes written.
|
|
// If the buffer is too small, PutVarint will panic.
|
|
func PutVarint(buf []byte, x int64) int {
|
|
ux := uint64(x) << 1
|
|
if x < 0 {
|
|
ux = ^ux
|
|
}
|
|
return PutUvarint(buf, ux)
|
|
}
|
|
|
|
// Varint decodes an int64 from buf and returns that value and the
|
|
// number of bytes read (> 0). If an error occurred, the value is 0
|
|
// and the number of bytes n is <= 0 with the following meaning:
|
|
//
|
|
// n == 0: buf too small
|
|
// n < 0: value larger than 64 bits (overflow)
|
|
// and -n is the number of bytes read
|
|
//
|
|
func Varint(buf []byte) (int64, int) {
|
|
ux, n := Uvarint(buf) // ok to continue in presence of error
|
|
x := int64(ux >> 1)
|
|
if ux&1 != 0 {
|
|
x = ^x
|
|
}
|
|
return x, n
|
|
}
|
|
|
|
var overflow = errors.New("binary: varint overflows a 64-bit integer")
|
|
|
|
// ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
|
|
func ReadUvarint(r io.ByteReader) (uint64, error) {
|
|
var x uint64
|
|
var s uint
|
|
for i := 0; ; i++ {
|
|
b, err := r.ReadByte()
|
|
if err != nil {
|
|
return x, err
|
|
}
|
|
if b < 0x80 {
|
|
if i > 9 || i == 9 && b > 1 {
|
|
return x, overflow
|
|
}
|
|
return x | uint64(b)<<s, nil
|
|
}
|
|
x |= uint64(b&0x7f) << s
|
|
s += 7
|
|
}
|
|
}
|
|
|
|
// ReadVarint reads an encoded signed integer from r and returns it as an int64.
|
|
func ReadVarint(r io.ByteReader) (int64, error) {
|
|
ux, err := ReadUvarint(r) // ok to continue in presence of error
|
|
x := int64(ux >> 1)
|
|
if ux&1 != 0 {
|
|
x = ^x
|
|
}
|
|
return x, err
|
|
}
|